1. Field of the Invention
This invention relates to a semiconductor memory device, and more specifically relates to an electrically rewritable and non-volatile semiconductor memory device (EEPROM).
2. Description of the Related Art
A NAND-type flash memory is known as one of EEPROMs. There is increased a demand for NAND-type flash memories in accordance with increasing of the demand for mobile devices, which deal with large capacitive data such as a still or moving image. Being formed of NAND strings (NAND cell units), each of which has multiple memory cells connected in series, a NAND-type flash memory has such features that the unit cell area is small; and it is easy to make the capacity large. While, there is such a drawback that the NAND-type flash memory is not suitable for a high-speed random access-use because the cell current is small.
In consideration of this, the data transmission rate is made high in such a way that data are read out to a page buffer, and then serially output, thereby resulting in that NAND-type flash memory is made adaptable to a high-speed system via a buffer DRAM and the like.
A sense amplifier usually used in the NAND-type flash memory is a voltage detecting type one, which precharges a bit line, and then detects the bit line voltage after discharging it with a cell current for a certain time.
By contrast, there has already been provided a differential sense amplifier of a current sensing type, which is possible to sense a small cell current at a high rate (for example, refer to JP-P2005-285161A). By use of this sensing scheme, in which a pair of bit lines becomes differential inputs, it is also possible to avoid interferences due to capacitances between bit lines.
On the other hand, in accordance with the large capacitive integration of the cell array, the time constant of a word line is made large more and more. Therefore, to effectively use the ability of the high-speed sense amplifier, it is required of a row decoder to be able to drive a word line, which is to be applied with a high voltage, at a high rate. There has been described, for example, in JP-P2002-63795A such a row decoder configured to be suitable for a NAND-type flash memory that has a voltage switching circuit with a high speed and high breakdown voltage performance.